can you help me? I have a fluid simulation in 2D, where I compute by finite differences on the grid the velocity field and pressure. So this is my output from the program. Now I have to show how the vertical component of traction on the boundary looks like. But I am not sure what the word traction means, what I have to compute. I can compute the stress from the velocity field as

$$
\sigma_{xy}=\eta (\frac{dv_x}{dy}+\frac{dv_y}{dx})
$$

$$
\sigma_{xx}=2\eta \frac{dv_x}{dx}
$$

$$
\sigma_{yy}=2\eta \frac{dv_y}{dy}
$$

So this I would know how to compute from my results - by finite differences.
What does the vertical traction mathematically means, what I have to compute?<br/>
Many thanks

Thsi is about traction:<br/>
"Note a convention that we have implicitly established: the sense of the
force per unit area $t$ across the oblique surface is that the fluid penetrated
by the unit normal $n$ acts on the fluid on the other side of the surface. This
force per unit area is called the traction across the surface. Note further that
$t_x$ indicates the traction across the y − z plane, not the x component of a
traction. Representation of components requires a second subscript: $T_{xy}$ is
the y component of the traction $t_x$ across the y − z face of the tetrahedron,
whereas $T_{xy}$ is the x component of the traction ty across the x − z face.
Thus, the first subscript represents the vector component of the force and
the second subscript indicates the face on which the force is acting.
"</br>
But how to compute it?